How to get a class object to store another class object in c#?

Suppose we have the C# code below that stores an instance of class A in class B. I want to create a version of class B that stores its own A. So when we change A outside of B, the internal A object in B doesn't change.

class A
{
    public int data = 9;
}
class B
{
    public B(A a_) { a = a_; }
    public A a;
}

class Program
{
    static void Main(string[] args)
    {
        A foo = new A();
        foo.data = 20; // this changes the field a in B as well
        B bar = new B(foo);

        Console.WriteLine(bar.a.data); // prints 20 instead of 9
    }
}

I can avoid this problem in C++ by using a templated bridge class that clones A when passing A into B. But apparently deep clones doesn't seem as widely used in C# for some reason(why?). Can anyone provide a good way around this storing problem.

C++ code:

template <typename T>
class Wrapper{
public: 
    Wrapper(const Wrapper<T> &orig){if(orig.data!=nullptr)data = orig.data->clone();}
    Wrapper(const T &origdata){data = origdata.clone();}
    Wrapper & operator=(const Wrapper<T> &orig){...}
    ... 
    ~T(){delete data;}
private:
    T *data;
}

class A{
public: 
    A():data(9){}
    int data;
    A *clone(){return new A(*this);}
}

class B{
public: 
    B(const Wrapper<A>& a_):ainner(a_){}
    Wrapper<A> ainner;
}

int main(){
    A a;
    B b(a);
    a.data =20;
    std::cout << b.ainner.data; // prints 9 still
    return 0;
}